(a) Field of the Invention
The present invention relates to a process for the vaporization of a liquid in first ducts or vaporization ducts, which are substantially vertical, and are opened at the top and bottom, of a plate type heat exchanger, by heat exchange with a main calorigenic fluid which circulates downwardly in second ducts of the exchanger. The invention may be used, for example, for the vaporization of cryogenic liquid, in particular liquid oxygen at the bottom of low pressure columns of apparatuses for the distillation of air with double column.
(b) Description of Prior Art
Plate type heat exchangers, which are largely used in the apparatuses for air distillation, comprise an arrangement of a plurality of vertical rectangular plates which are made of aluminum and define therebetween a large number of flat ducts of large dimensions. Waves which are used as braces between the thermic plates and wings, are mounted between the plates. The edges of the ducts are closed by means of bar-cross-braces, and breaks therebetween enable the introduction of the fluids into the exchanger and their removal by means of semi-cylindrical inlet and outlet boxes which are fixedly mounted opposite these breaks.
Among these exchangers, some of them, so called "bath type vaporizers", include vaporization ducts which are completely opened at the top and bottom. They are currently used, for example, to vaporize liquid oxygen by condensation of mean pressure nitrogen at the bottom of low pressure columns of double columns for air distillation.
Such a bath type vaporizer is immersed in the bath of fluid to be vaporized (oxygen in this case of a double column). The circulation of this fluid is made possible by thermosiphon effect.
For a given exchanger and heat flow, the total flow which circulates in the vaporization ducts, depends on the degree of submergence, which is the ratio between the height of immersion of the exchanger in the bath of liquid and the height of the exchanger in %. This flow decreases when the submergence decreases, and the recirculation (ratio between the liquid flow at the outlet and the vaporized flow) is annulled for degrees of submergence which are too low, thus causing a drying in the upper part of the vaporizer.
In certain cases, and for example in the above mentioned oxygen vaporizers, such an operation under dry condition is not permitted, for safety reasons. As a matter of fact, there is a risk of deposit and concentration of hydrocarbons which may react with oxygen to cause an explosion. One is therefore forced to operate with relatively high submergences, generally of the order of 70 to 80%.
This is detrimental on the point of view of the performance of the exchanger, since the hydrostatic height of the bath of liquid results in a sub-cooling of the liquid at the bottom of the exchanger, this sub-cooling reaches for example 0.8.degree. C. for a height of liquid oxygen of 1 meter in a low pressure column operating under 1.3 bar absolute. The lower part of the vaporizer is therefore used to warm this liquid and to bring it to its boiling point, and this zone may reach an important fraction of the height of the vaporizer (1/3 to 1/4 for a submergence of 100%).
Under these conditions, it is difficult to operate a bath type oxygen vaporizer with a small temperature gap lower than 1.degree. C. between oxygen and means pressure nitrogen, unless the height of the vaporizer is reduced and exchangers placed in superposed baths are mounted at the bottom of the column. This solution has already been used with certain apparatuses, however it represents a costly investment.